The present invention relates generally to methods and apparatuses for handling a stream of molten metal and more particularly to methods and apparatuses for preventing the entry into the molten metal stream of gas from the surrounding atmosphere.
In conventional casting operations, a vertically descending stream of molten metal (e.g., molten steel) flows from an upper container such as a ladle to a lower container such as the tundish of a continuous casting apparatus. The stream typically flows through a vertically disposed nozzle having an upper end communicating with a bottom outlet from the ladle and a lower end disposed above the top surface of a molten metal bath in the lower container, e.g., the tundish. In the absence of protective measures, that portion of the molten metal stream between the lower end of the nozzle and the top of the molten metal bath is exposed to the outside atmosphere surrounding the stream, e.g., air. In such a case, air can be absorbed into the stream which is undesirable because it introduces oxygen and nitrogen as impurities into the molten metal. To prevent this from occurring, it has been conventional to enclose that part of the descending metal stream, below the lower end of the nozzle, within a vertically disposed, tubular shroud having a lower end submerged within the molten metal bath in the lower container.
The shroud is aligned with the nozzle and has an upper portion which surrounds and removably engages the lower portion of the nozzle at a junction of the two. The interior of the tubular shroud typically has a cross-sectional area (or diameter) greater than the cross-sectional area (or diameter) of the nozzle's interior. Because the shroud has a larger interior cross-section than the nozzle, a descending molten metal stream which fills the entire interior cross-section of the nozzle will not fill the entire interior cross-section of the shroud. As a result, the molten metal stream descending from the nozzle through the interior of the shroud will create a partial vacuum in the shroud. There is a seam where the upper portion of the shroud removably engages the lower portion of the nozzle at their junction, and the partial vacuum created within the interior of the shroud has a tendency to aspirate outside air from the atmosphere surrounding the shroud and the nozzle into the interior of the shroud through the seam at the junction. This is undesirable because it will introduce oxygen and nitrogen into the molten metal stream.
Attempts have been made in the past to prevent outside air from being aspirated into the interior of the shroud, but none of these attempts has been sufficiently successful. For example, in one attempt, an annular clearance was provided between the upper portion of the shroud and the lower portion of the nozzle, at the location of the seam, and an inert gas, such as argon, was continuously introduced into the clearance to exclude air from entering the clearance. The inert gas was drawn from the clearance into the shroud by the partial vacuum in the shroud, and this necessitated replenishment of the inert gas in the clearance.
A problem with the expedient described above was the occurrence of eddy currents in the inert gas in the clearance. This allowed the periodic escape of the inert gas from the clearance to the atmosphere and the entry of outside air into the clearance. This air was drawn from the clearance into the interior of the shroud where it could mix with the descending stream of molten metal. In addition, the escape of the inert gas to the atmosphere was wasteful and necessitated too great a replenishment of the inert gas in the clearance.